1. Technical Field
The present invention relates to an optical modulator using an isolator and an optical transmitter including the same. More particularly, the invention relates to an optical modulator for modulating a continuous wave optical beam by provoking optical attenuation by varying a magnetic field applied to an isolator, and an optical transmitter including the same.
2. Related Art
An optical modulator modulates a continuous wave optical beam into an optical signal which has the same form as an electrical signal. When several wavelength optical signals are simultaneously transmitted by a wavelength division multiplexing (WDM) system, their center wavelengths are jittered by the chirping property of an optical signal spectrum which is generated by applying an electrical signal, thus degrading the transmission characteristics. In order to solve this problem, a laser diode is driven to output a continuous wave, and a beam output by the laser diode is modulated. Optical modulation is accomplished by an external modulator such as an electro-optic modulator or a lithumnaobate (LiNbO.sub.3) modulator.
FIG. 1 is a block diagram illustrating the configuration of a conventional optical modulator. Referring to FIG. 1, the optical modulator comprises a laser diode 100, first optical waveguide 102, second optical waveguide 104, an electrode plate 106, and an electric signal source 108.
The operation of the above elements will now be described. First, a continuous optical signal applied from the laser diode 100 is coupled into the optical waveguides 102 and 104. When an electrical signal is applied by the electrical signal source 108 to the electrode plate 106, the refractive index of the first optical waveguide 102 is changed. The changed refractive index changes the propagation constant of the first optical waveguide 102, and the changed propagation constant changes the phase of an optical signal passing through the first optical waveguide 102. Accordingly, there is a phase difference between the phase-changed optical signal of the first optical waveguide 102 and a non-phase-changed optical signal of the second optical waveguide 104.
The output optical signal of the optical modulator is modulated according to this phase difference. That is, when the two phases are the same, the output optical signal is intensified, but when the two phases have a difference of 180.degree., the output optical signal is canceled out. In this way, the output optical signal is turned on/off (modulated) at an output port. The extinction ratio in an off state is about 22 dB.
However, in this optical modulator, insertion loss is high (generally, about 6 dB), and a change in its characteristics due to polarization is severe. Also, this modulator requires a high reference voltage and an electrical modulation signal. This modulator also requires a heat release process since internal heat becomes high when such a high voltage is applied.
In a low-cost optical modulation technique for a subscriber network, a spectrum sliced source using amplified spontaneous emission of an optical amplifier is used as a continuous wave optical source, and a method of modulating an electrical signal into an optical signal using the aforementioned external modulator is adopted. In this case, more costs are required in order to include a subscriber network, due to the high cost of an expensive optical modulator.
Thus, as described above, optical modulators are burdened by the disadvantages of high insertion loss and severe changes in their characteristics due to polarization. In addition, such modulators require a high reference voltage, an electrical modulation signal, and a heat release procedure. Therefore, there is a need for an optical modulator which does not suffer the above disadvantages and other disadvantages disclosed herein.